Quantum transport in topological semimetals

Topological semimetal is a three-dimensional topological state of matter, in which the conduction and valence energy bands touch at a finite number of Weyl nodes. The Weyl nodes always appear in pairs, and carry opposite chirality and linear dispersion, much like 3D analogue of graphene. Topological semimetals also host paired monopoles of Berry curvature in momentum space and Fermi arcs.In this talk, I will cover several quantum transport effects that have been observed recently in topological semimetals. (1) Weak antilocalization, which can give rise to a negative magnetoconductivity proportional to the square root of magnetic field at low temperatures. We have a systematic theoretical study on the weak antilocalization in topological insulator, 2D materials, and topological semimetals. The theory has been applied in recent experiments on the topological Weyl semimetal TaAs. (2) Chiral anomaly, which is expected to give a positive quadratic-B magnetoconductivity (i.e., negative magnetoresistance) in parallel magnetic fields. We have experimentally observed the effect in the topological semimetal Cd3As2. (3) The chiral anomaly is also predicted to give a linear-B magnetoconductivity in the quantum limit at high fields. However, all experiments on Weyl and Dirac topological semimetals show a negative magnetoconductivity in high fields. We show that the high-field positive magnetoconductivity may not be a compelling signature of the chiral anomaly and will be helpful for interpreting the inconsistency in the recent experiments and earlier theories.